Silicon Carbide (SiC) has been a material of interest for microelectronics for a considerable time. The large bandgap and chemical stability enables it to operate in environments where silicon can not function. Silicon electronics cease to function properly if the operating temperature exceeds 350° C. whereas SiC based devices have been shown to operate at temperatures as high as 650° C. The chemical inertness of SiC has generated interest in applications for chemical and biological sensors. Current uses for SiC based electronics largely focus on power applications where the high dielectric breakdown strength and temperature characteristics make it stand out. Applications in non-power applications have been sporadic with the most notable being their use in high frequency transistors, resulting in higher mobility and higher power density, for military applications and as the original blue LED, nearly a decade before indium nitride came on the scene.
Many advanced microelectronic devices require the removal of surrounding materials from the planar substrate surface. Field effect transistors based on the wrap around gate design require the channel to be exposed to the gate not only on the top but also on the sides, leading to lower off state leakage current. For example, Micro- and Nano-Electro Mechanical Systems (MEMS and NEMS respectively) require structures that are detached from the substrate to form moving mechanical components. In addition, SiC has promising optical properties in the mid infrared band and could potentially serve as a waveguide in that frequency range. These applications all require the ability to form structures with well defined edges.
One of the impediments to the wider use of SiC is the difficulties involved in etching to produce devices. The most common wet etching technique uses molten potassium hydroxide, at a temperature of ˜400° C. This is highly undesirable for several reasons. The presence of potassium would require stringent isolation from any silicon process line. Potassium, along with the other alkali metals are well know for diffusing into silicon and creating what are known as deep level charge traps. These traps degrade the performance of any advanced digital circuit. In addition, the etching process requires a mask that is chemically resistant to the etchant yet be easily removed after the etch. In the case of molten potassium hydroxide, the choice of mask material is reduced to the noble metals or carbon, none of which makes for a practical material. The chemical stability of SiC has precluded any reasonable wet etching option, leaving the more expensive options of dry etching via reactive ion etching and inductively coupled plasma etching.